This invention relates to an improved coated synthetic paper and the method of making the same. More particularly, this invention relates to an improved coated synthetic paper formed of polyolefinic synthetic pulp having on its surface a coating layer adapted for offset printing.
Polyolefin synthetic pulp is advantageous and superior in that it has a good heat sealability, higher strengths both in dry and wet conditions and a good dimensional stability which can never be obtained with natural cellulose pulp. Some attempts have been made to provide a pigment coated synthetic paper in which these advantages of the polyolefin synthetic pulp are utilized. For example, Japanese Pat. Publication No. 29,607 of 1972 (open to inspection) discloses a coated synthetic paper having a coating layer formed of inorganic pigments such as clay, calcium carbonate, satin white talc and the like, and adhesives such as casein, starch, styrenebutadiene latex and the like on a base paper formed either solely of polyolefin synthetic pulp or of a mixture of polyolefin synthetic pulp with natural cellulose pulp and/or another synthetic pulp. However, such coated synthetic paper like this is disadvantageous in that ink transfer cannot properly be made especially in case of the multi-color offset printing. Generally, in the multi-color offset printing, the fountain solution remains on the surface of the sheet during the printing process if the surface of the coated paper is poor in absorbing the fountain solution. The existence of the fountain solution on the surface of the paper causes to prevent the receipt by the paper of the ink to be transferred from a blanket cylinder to the paper surface. That is, when an aqueous coating composition consisting essentially of a pigment component and an adhesive component is applied onto the base paper formed of polyolefinic synthetic pulp and the coated paper is then dried, the above-mentioned coating composition penetrates into the base paper more slowly than in case of the base sheet formed of natural cellulose pulp. Therefore, the adhesive contained in the coating composition migrates toward the surface of the coated layer of the sheet in the process of drying, and the adhesive in the resultant coating layer is distributed more densely near the surface. This would be the reason why the coated synthetic paper is not good in the absorption of the fountain solution. Even if the amount of the polyolefinic pulp used is relatively small, the failure of absorption of the fountain solution occurs locally. If the amount of the polyolefinic pulp used is relatively large, the failure of absorption of the fountain solution occurs substantially over the whole surface of the paper.
On the other hand, in view of convenience of handling and reduction of cost of mailing and shipping it is desired to reduce the weight of the pigment coated paper. If the thickness of a base paper is reduced for this purpose, then it will result in deterioration of the strength, the brightness and the opacity of the paper. It has been hardly possible to avoid the above-mentioned disadvantages with use of the conventional natural cellulose paper having a pigment coating thereon.
The primary object of this invention is to provide an improved coated synthetic paper adapted for offset printing.
Another object of this invention is to provide an improved coated synthetic paper having a high opacity, good brightness and good dimensional stability.
A further object of this invention is to provide an improved coated synthetic paper which is relatively light but stiff enough.
A still further object of this invention is to provide an improved method for producing the above-mentioned coated synthetic paper.
Other objects and advantages of the invention will become apparent from the following detailed description of the invention.